This chapter reviews the mechanisms of temporal gene regulation utilized during the three main stages of the developmental cycle, as well as how transcription is silenced in elementary body (EB). It begins with midcycle because the majority of chlamydial genes are regulated during this stage of reticulate body (RB) growth and replication. The transcriptional regulation of midcycle genes, late genes, EBs, and early genes in sequence, mirroring the events that occur during replication, conversion of an RB into an EB, and then back again into an RB at the start of a new round of infection are discussed. Alternative sigma factors have important roles in the regulation of virulence gene expression in many bacteria. A study showed that a subset of early genes resemble midcycle genes in having promoters that are supercoiling responsive. There is emerging evidence that gene expression during the chlamydial developmental cycle is also regulated by small RNAs (sRNAs). Two complementary genome-wide studies have since identified many more sRNAs in C. trachomatis. Chlamydia spp. are likely to encode more sRNAs because neither study examined chlamydial RNA before 24 hpi, and both would therefore have missed sRNAs that are only present at earlier times.

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